Take me home

The amazingly amazing amaze of lazy data structures

Published December 14, 2019

This post is part of a series: Advent Calendar 2019

Two things makes me keep coming back to Clojure: Persistent data structures and lazy sequences.

This post is about lazy sequences.

Lazy fibonacci

To demonstrate lazy sequences, let's implement the Fibonacci sequence.

Here's a JavaScript version of the Clojure code to follow, so you have something to compare it to.

// JavaScript version, for good measure
function fib(n, xs) {
  xs = xs || [0, 1]
  
  const c = xs.length
  
  if (c < n) {
    return fib(n, xs.concat([
      xs[xs.length - 1] + xs[xs.length - 2]
    ]))
  } else {
    return xs;
  }
}

fib(10)
// [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

Here's the Clojure version of the same code.

; "Plain" version
(defn fib
  ([n] 
   (fib n [0 1]))
  ([n xs]
   (let [c (count xs)]
     (if (< c n)
       (fib n
         (conj xs 
           (+ (nth xs (- c 1))
              (nth xs (- c 2)))))
       xs))))
       
(fib 10)
; [0 1 1 2 3 5 8 13 21 34]

All good, right?

Well, it's not the prettiest code in the world.

Let's fix that, by making literally the prettiest code in the world.

; Lazy version - literally the prettiest 
; code in the world!
(defn fib
  ([]
   (fib 0 1))
  ([a b]
   (lazy-seq (cons a (fib b (+ a b))))))
   
(take 10 (fib))
; (0 1 1 2 3 5 8 13 21 34)

Oh yass.

First of all, that's much less code.

Second of all, you can make it use BigDecimals and get (fib 100000) super fast, with good memory usage and no stack overflow and all that jazz.

; Clojure uses long by default, so make it use 
; bigdec for arbitrarily sized numbers
(defn fib
  ([]
   (fib (bigdec 0) (bigdec 1)))
  ([a b]
   (lazy-seq (cons a (fib b (+ a b))))))

(time (last (take 100000 (fib))))
; "Elapsed time: 299.1678 msecs"
; 1605285768272[...20880 digits (yes,
; really) cut for brevity]790626M

That's pretty cool.

By the way, the threading macro is awesome. You use it to write statements in the order that they are called.

; Plain version
(time (last (take 100000 (fib))))

; Awesome threading macro version
(->> (fib)
     (take 100000)
     (last)
     (time))

So what actually is this laziness?

Well, the whole idea is that you separate two concepts: generating data, and extracting/using data.

See how the first version had both iteration, data generation, checking if our max n was reached etc in the same code?

We don't need to do that with laziness.

With the lazy version, we just define what the data structure looks like. You get a "lazy sequence" back, and you can think of it as a view of an infinitely sized Fibonacci sequence, that is not actually computed until you actually ask for some data from the sequence.

So it's the call to (take) that actually makes something happen.

; "iterate" is also lay
(take 10 (iterate inc 0))
;  (0 1 2 3 4 5 6 7 8 9)

; So is "range"
(take 10 (range -50 9000 3))
; (-50 -47 -44 -41 -38 -35 -32 -29 -26 -23)

Lazy map

This code prints absolutely nothing.

(map (fn [n] (print n ""))
     [1 2 3 4 5 6 7 8 9 10])

; Nothing.. Absolutely nothing

That's because (map) is also lazy. We have to actually get some items from it for the map function to be called.

; Wait what, now?
(->> (map (fn [n] (print n "") n) 
          [1 2 3 4 5 6 7 8 9 10])
     (take 3))
     
; prints
; 1 2 3 4 5 6 7 8 9 10

; returns
; (1 2 3)

Surprised? We ask for 3 items, but all 10 items seems to be printed! Is it not lazy after all?

It turns out that under the hood, Clojure will some times do a performance optimization and realize lazy sequences in batches of 32.

; Batching
(->> (map (fn [n] (print n "") n)
          [1 2 3 4 5 6 7 8 9 10 11 ... 34 35 36])
     (take 3))
     
; prints
; 1 2 3 4 5 6 7 8 9 10 11 ... 30 31 32 

; returns
; (1 2 3)

Note: it will run the mapping function 32 times and "pre-cache" the lazy results - but you'll still get a sequence with 3 elements in it returned from (take).

But, if your source is a lazy sequence and not a vector, it won't do that.

; No batching when source is lazy
(->> (map (fn [n] (print n "") n) 
          (iterate inc 1))
     (take 3))

; prints
; 1 2 3

; returns
; (1 2 3)

Generating PIN codes

This one is pretty cool. Let's say you want to generate N random 4 digit pin codes.

You can generate an infinite lazy sequence of random numbers:

; Possible duplicates!
(->> (repeatedly #(rand-int 9999))
     (take 5))

; (643 6000 483 8668 7493)

But this list can contain duplicates! The chances are low when you just get 5 items obviously, but that doesn't matter - our requirement was that the list should be unique.

Enter the awesomeness of (distinct). That function will take a list, and return a list with no duplicates - and it is lazy!

; No duplicates!
(->> (repeatedly #(rand-int 9999))
     (distinct)
     (take 5)
     ; Also format it, for good measure
     (map #(format "%04d" %)))

; ("6856" "3461" "8833" "4884" "0004")

So good. So, so very good.


Questions or comments?

Feel free to contact me on Twitter, @augustl, or e-mail me at august@augustl.com.